The effect of rare earth elements on phosphorus leaching in intact soil columns

Abstract

Phosphorus (P) has been linked to eutrophication in surface waters because it is a limiting nutrient for algal growth, and recent studies have shown that P transport through subsurface flow is of growing concern. In this study we amended dairy and poultry manures with lanthanum (La) and ytterbium (Yb) chlorides to assess the efficacy of using rare earth element (REE) chloride amendments in reducing P leaching through intact soil columns and to determine the major pathways of applied P leaching. Significant reductions in dissolved P (DP; 56% to 64%), particulate P (PP; 22% to 36%), and total P (TP; 41% to 51%) in leachate were seen when dairy manure was amended with REE-chlorides, but no significant reductions in these P fractions were seen in amended poultry litter. Differences in P leaching losses between the two REE-amended manures were likely due to better mixing and dissolution of the REE-chlorides and better precipitation of an insoluble particulate REE-phosphate salt in the liquid manures prior to being applied. Very little vertical transport of REEs was observed in soil leachate over repeated events. Elevated concentrations of REEs along soil macropores at depths greater than 15 cm (5.9 in) suggest the importance of this pathway. However, due to the extremely low concentrations of REEs found at depth and the much higher values of soil P, the ability to "label" the manures with REEs to track P through macropores in the soil was only suggestive and by no means conclusive. Results point to the efficacy of REE-chlorides in lowering P solubility in liquid manures but limited potential in tracking subsurface transport pathways of applied manure P in soils.Key words: dairy manure-leaching-lysimeters-phosphorus-poultry litter-rare earth elements Once relegated as a marginal pathway of agricultural phosphorus (P) loss, leaching of P through soils has increasingly been associated with some of the world's most prominent examples of eutrophication (Sharpley et al. 1994). From Lake Erie, to the Chesapeake Bay, to the Baltic Sea, P leaching through the profile of agricultural soils fertilized with either mineral or manure sources of P has been tied to surface water P loadings. An increase in the enrichment of ground waters with P has been attributed to surface applications of P in agriculture (Holman et al. 2008;Domagalski et al. 2011). Increasingly, demand has grown for techniques that elucidate the mechanisms of subsurface agricultural P losses (Djodjic et al. 2002) and opportunities for controlling these losses .It is widely recognized that subsurface P transfers occur through macropore pathways in which P in leachate bypasses sorption sites within the matrix of a soil (Djodjic et al. 2002; Sørensen and Jensen 2013). While leaching of dissolved P has been the overwhelming focus of many studies, a consistent observation is that substantial fractions of particulate P can be leached. In soils with artificial drainage (e.g., tile drainage), observations of turbid drainage discharge are common (Frey et al. 201...